nspawn: Drop CAP_NET_BIND_SERVICE if in userns with identity mapping (#38723)

Even if there's no uid shift, we still won't be able to bind to
privileged ports in the host network namespace, so drop the capability
regardless of whether we have a uid shift or not.

src/nspawn/nspawn.c

@@ -5950,7 +5950,7 @@ static int run(int argc, char *argv[]) {
         /* If we're not unsharing the network namespace and are unsharing the user namespace, we won't have
          * permissions to bind ports in the container, so let's drop the CAP_NET_BIND_SERVICE capability to
          * indicate that. */
-        if (!arg_private_network && arg_userns_mode != USER_NAMESPACE_NO && arg_uid_shift > 0)
+        if (!arg_private_network && arg_userns_mode != USER_NAMESPACE_NO)
                 arg_caps_retain &= ~(UINT64_C(1) << CAP_NET_BIND_SERVICE);
 
         r = verify_arguments();